The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
An important class of carbon nanotubes is single-walled carbon nanotubes (SWCNTs). They are generally produced as ensemble samples containing both metallic (m-SWCNTs) and semiconducting (sc-SWCNTs) nanotubes with a distribution of chiralities centered at a mean diameter. Several methods can be used to produce SWCNTs which will vary in the distribution of chiralities, diameter range, semiconducting/metallic (sc/m) content and average length. For many applications, such as thin film transistors (TFTs), a sc-purity higher than 99% is needed.
Several methods have been used to demonstrate the effective enrichment and isolation of sc-SWCNTs with greater than 99% sc-purity as assessed by absorption spectroscopy. Among these methods are density gradient ultracentrifugation (DGU), gel chromatography (GC), dielectrophoresis and selective extraction by conjugated polymers. Amongst these listed options, the simplicity of the conjugated polymer extraction (CPE) process, which generally entails a dispersion followed by a centrifugation step, further distinguishes it from the rest as a cost-effective method for the isolation of sc-SWCNTs with greater than 99% semiconducting content.
The first disclosure that conjugated polymers could selectively disperse semiconducting SWCNTs and lead to enriched semiconducting SWCNT fractions of relevance for thin film transistor fabrication can be found in the patent literature (Malenfant 2007). Subsequently, the exceptional selectivity that could be achieved with polyfluorene derivatives towards specific semiconducting SWCNT chiralities was demonstrated. More recently an effective enrichment of HiPco sc-SWCNTs using poly(3-dodecyl thiophene) (P3DDT) and arc-plasma-jet tubes using PFDD was also demonstrated to provide TFTs with mobilities greater than 10 cm2/Vs. Collectively, these results, amongst others, have clearly shown the potential for conjugated polymers in sc-SWCNT enrichment and TFT device fabrication.
To date, many homo- and copolymers of phenylenevinylene, carbazole, thiophene and fluorene have been investigated for enrichment. For example, P3DDT displayed a promising result in the separation of HiPCO nanotubes, however P3DDT is not suitable for the separation of larger diameter SWCNTs, which are more desirable when trying to minimize contact resistance and to obtain a large electron mobility in thin film transistors. Similarly, it has been observed that poly(9,9-dioctylfluorene) (PFO) has a high selectivity in dispersing small-diameter sc-SWCNTs with large chiral angles (20°≤0≤30°), but not large-diameter SWCNTs, which is believed to be difficult to disperse and to enrich owing to the strong interaction between the nanotubes associated with the low curvature of the nanotube wall. As a result, co-monomer units have been introduced into the polyfluorene main chain in order to target specific tube chiralities/diameters. They include: phenylene-1,4-diyl, thiophen-2,5-diyl, anthracene-9,10-diyl, anthracene-1,5-diyl, naphthalene-1,5-diyl, 2,2-bithiophene-5,5′-diyl, and benzo-2,1,3-thiadiazole-4,7-diyl.
Furthermore, the length of the side alkyl chain of the polymers has a significant impact on the enrichment effectiveness. Polymers with 12-carbon long side chains showed an improved selectivity to sc-SWCNTs with larger diameters. Recently work on the enrichment of large diameter SWCNTs using fluorene homopolymers with long alkyl side chains has been done, which achieved a device performance of 14.3 cm2/Vs and on/off ratio over 105.
The basic CPE process is described in, for example, WO 2015/024115 and J. Ding et al. (Enrichment of large-diameter semiconducting SWCNTs by polyfluorene extraction for high network density thin film transistors, Nanoscale, 2014, 6, 2328-2339), both of which are incorporated herein by reference.
CPE and its deduced hybrid process (disclosed, for example, in WO 2015/024115) have proved to be an efficient technique for a large scale sc-SWCNTs enrichment at a purity higher than 99.9%. However, removal of the polymer that wraps and disperses the SWCNTs is a tedious process.
One method is the use of excess polymer to boost the yield of the CPE process. However, this leads to long filtration times (for example, it takes more than 24 h to filter 1 L of sample having 50 mg of SWCNTs), along with higher polymer/tube rations in the final product. Removal of the excess polymer typically requires long soaking treatments in solvent, followed by additional filtration, which is both unproductive and costly.
US 2012/0104328 discloses a conjugated polymer (CP) process for selectively separating sc-SWCNT from m-SWCNT. The conjugated polymers are polythiophenes. Paragraph [0093] indicates that an additive may be used to improve separation efficiency and that the additive can be ethylene diamine tetraacetic acid (EDTA).
Gao J, et al. Carbon. 49(1), January 2011, 333-338. discloses the use of amine-functionalized polyfluorenes, specifically a dimethyl amino-substituted polyfluorene, in the extraction of sc-SWCNT. The amine-substituted polymer provides: better solubility (faster dissolution) of the SWCNT than PFO; higher total amounts of dispersed SWCNT; and a lower ratio of CP to CNT, reducing the ratio from 15:1 (found with PFO) to a ratio in a range of 10:1 to 1:1.
A number of publications (U.S. Pat. Nos. 7,250,569; 6,331,262; 8,231,854; 2010/0278714; Hersam M. C., Nature Nanotechnology, Vol. 3, July 2008, 387-394; Hwang J-Y, et al. J. Am. Chem. Soc. 130(11), 2008, 3543-3553; and Samsonidze G. et al. Applied Physics Letters, 85(6), August 2004, 1006-1008) disclose that amines of various sorts can be used to aid in the selective separation of SWCNT or in the solubilisation of SWCNT. However, none of these documents disclose that the amine can be used in a CPE process.
There is a need for an improvement in a CPE process that selectively disperses sc-SWCNT in a solvent, so as to facilitate removal of the conjugate polymer that wraps and disperses the sc-SWCNTs.